LoS MIMO (Line-of Sight Multiple-Input-Multiple-Output) technology is used to increase the capacity of fixed microwave links utilizing the spatial diversity of multiple antennas transmitting independent data streams in the same frequency band.
Input data streams are normally modulated at the transmitter side using QAM (Quadrature Amplitude Modulation), which is a digital modulation/demodulation scheme. An amplitude-shift keying suppressed carrier (ASK-SC) digital modulation scheme is preferably used. The digital data streams are conveyed by modulating the amplitude of two carrier waves. The waves are out of phase with each other by 90 degrees and are called quadrature carriers/components. The modulated waves are summed and the result waveform is a combination of both phase-shift keying (PSK) and ASK.
The task of the LoS MIMO receiver is the separation of the input data streams at the receiver side by equalizing the effect of the microwave channel. When demodulating the waveform, phase tracking is a known method used in QAM. A carrier recovery loop is often used for enabling a coherent demodulation of high order QAM constellations. In order to improve the phase tracking performance, “pilot” symbols of lower constellation order are periodically inserted, such as for instance 4 QAM symbols. Pilot symbols of higher constellation order may be used for higher order QAM constellation demodulation. For synchronization, the best known method uses the pilot signal and FFT (Fast Fourier Transformation) to determine frequency deflections, but it is time and power consuming.
There are several know equalizer algorithms, such as LMS (Least Mean Square), CMA (Constant Modulus Algorithm) and MCMA (Modified Constant Modulus Algorithm). However, these algorithms operate properly only if the local oscillator frequencies are identical at transmitter and receiver side respectively (frequency locking). If this is not provided, the equalizer has to be amended with appropriate synchronization algorithms. For synchronization, the best known method uses pilot signal and FFT to determine frequency deflections, but it is time and power consuming.
There are existing receiver architectures comprising two PLL's and an equalizer, but locking for the correct spectrum lines can only be assured manually.
Consequently, there are no fully automated solutions for locking at the correct spectrum lines and pilot signals needs to be used. Moreover, FFT is time and computation power consuming. Finally, equalization often needs complicated matrix operations.